Prospects of Nb 3 Sn performance enhancement for high field fcc applications

1. Prospects of Nb3Sn performance enhancement for high field fcc applications Mike Sumption, X. Xu, and E.W. Collings Center for Superconducting and Magnetic Materials, MSE, The Ohio State University Hyper Tech Research, Inc. Xuan Peng Mike Tomsic Selected data from SupraMagnetics, Inc. LeszekMotowidlo This work was supported by the U.S. Department of Energy, Office of Science, Division of High Energy Physics, under SBIR phase I DE-SC0013849 and University Grant DE-SC0011721.

2. Nb3Sn based Conductor Options Bruker PIT Conductor At 15 T Jc is 2700 A/mm2, JnonCu(RRP) = 1600 A/mm2, and Tube/PIT 1350 A/mm2

3. Need of FCC for improved Nb3Sn Needed conductor amount for FCC • FCC with 16 T magnets: 4,500 tons of Nb3Sn and 10,000 tons on NbTi • FCC with 20 T: 1,400 tons HTS, 6,300 tons Nb3Sn, 11,000 tons NbTi • Conductor Specifications? • Non Cu Jc of 1500 A/mm2 at 16 T? • Will need both ternary alloy and enhanced pinning

4. Possibilities for Nb3Sn Jc enhancement • Keep pinning the same but enhance Jc further from the presently optimized ternary (In principle maybe possible, but so far difficult) • Enhance pinning in Binary • Enhanced pinning in similar-to-present ternary alloy

5. Importance of refining Nb3Sn grain size Maximum pinning force Fp,max For present strands, reducing grain size leads to increase in Fp,max, but Fp,max is always at 0.2Bc2. Bc2 0.2Bc2 The high-field Jc can be significantly improved by shifting Fp,max to 0.5Bc2. Experiments (Dietderich, 1997) on films (electron-beam co-evaporation deposition): as grain size is very fine, the Fp-B curve peak shifts to 0.5Bc2. 50-100 nm 15-30 nm • How to obtain such small grain size in practical strands? • Reducing reaction temperature? Cannot go to < 80 nm. • Adding second phase particles to prevent grain coarsening? D. R. Dietderich and A. Godeke, Cryogenics 48, 331 (2008).

6. Refine Nb3Sn grain size by adding second phase particles Procedures to fabricate Nb3Sn: Billet of Sn, Cu, Nb alloy Extrusion, drawing Final-size strand A-B alloy Heat treatment JO The particles can only be formed during heat treatment. Superconducting Nb3Sn O source One method: internal oxidation. BOn • “Internal oxidation”: O diffuses in an A-B solid solution, and selectively oxidizes the solute B, forming BOn particles in matrix A. • Prerequisites: • B is much less noble than A. • O diffuses fast in matrix A. • O partial pressure is only high enough to oxidize B. C CO CB Zone II Zone I For the case of Nb3Sn, we can use Nb-Zr alloy, since Zr is much less noble than Nb.

7. Can the internal oxidation method work for Nb3Sn wires? To find out if this internal oxidation method really works for Nb3Sn strands, we made an experimental monofilament: 750 °C Reacted in pure Ar Intra-granular ZrO2 particles Cu stabilizer First, etch off the outside Cu to expose Nb-1Zr to the atmosphere. Newly-formed Nb3Sn grains Nb Nb-1 at.%Zr Earlier-formed Nb3Sn grains Cu Reacted in Ar-O • During heat treatment: • Pure Ar atmosphere: no oxygen supply • Ar-O mixture: sufficient O supply Sn core X. Xu et al., Appl. Phys. lett. 104, 082602 (2014) Note ZrO2 particles 10 nm OD • How do ZrO2 particles refine Nb3Sn grain size? • Impede Grains coarsening: distinct gradients in grain size. • Be nucleation centers: newly-formed grains in the internal oxidation samples are smaller. Inter-granular ZrO2 particles

8. Supplying oxygen to Nb-Zr alloy in real strands 1. Supply O externally during heat treatment? The diffusion of O in Cu layer: assuming CO(O) = Cs, CO(Nb) = 0. In this work we will use oxide powder as O source, because powder possesses good flowability during wire processing. The goal is to transfer the O from oxide powder to Nb1Zr during heat treatment. Cu O2 O2 O2 O2 ri Nb ro O2 O2 O2 Oxide and Nb-Zr do not have to contact. As long as the atmosphere is connected, O can be transferred via atmosphere. O2 Little O is absorbed by Nb-Zr. Oxide powders that can supply O to Nb: CuO, SnO2, ZnO, Nb2O5. Oxide powders that cannot supply O to Nb: NbO or NbO2.

9. An example tube type monofilament SnO2 powder: 625 °C: A control: NbO2, which does not supply O. A green-state subelement: Wires with NbO2 and SnO2: 650 C for 150 h: NbO2: < 0.3at.% O, SnO2: supplied >3 at.% O. Ave: 36 nm 200 nm 200 nm Average grain sizes: 104 nm and 43 nm. SnO2-650 °C / 400h: SnO2 NbO2 Cu • Nb-1Zr SnO2 The 12 T layer Jcof the state-of-the-art Nb3Sn strands is 5 kA/mm2, so the SnO2-625x800h doubles this record. Cu Sn core Hyper Tech- Ohio State application

10. Fp-B curves of Internal oxidation strands Fp,max vs reciprocal of grain size: Normalized Fp-B curves: • With the same Nb3Sn phase fraction Bc2 was obtained by fitting the Fp-B curve using Fp=Kbp(1-b)q. Two companies are using this technique to fabricate products. As grain size d is < 50 nm, reducing d shifts the curve both to the up and to the right. 0.25 mm

11. Suppose we can further refine grain size, what will be the Jc? Using Nb-2%Zr, 25 nm of grain size is likely reachable. A Nb-2%Zr sample reacted at 650 °C: Suppose grain size is refined to 20-25 nm with Bc2 kept at 25 T. 4 6 Average grain size: 34 nm. 1 State-of-the-art RRP wires with 4.2 K, 12 T non-Cu Jc of 3000A/mm2. With grain size of 25 nm and Bc2 = 25 T, what’s the Jc?

12. Observations and Plans Composition/Microstructure Factors we can control: chemistry/processing Properties Performance Nb3Sn phase fraction 1. Starting Nb/Sn/Cu ratio High-fieldJc 2. Precursor architecture Additions: Ta/Ti Bc2 3. Doping: Nb-Ta, Sn-Ti Sn content Fp,max 4. Heat treatment: temperature, time Grain size Peak field 5. Other approaches: e.g., internal oxidation Observations: Ic of Nb3Sn strands can be improved by improving: Nb3Sn fraction, Bc2, and Fp,max. There is still some room for Bc2 improvement by enhancing Sn at.%. A model has been developed on what determines Sn content of Nb3Sn and how to control it. There is huge room for Jc improvement via grain size refinement. An internal oxidation method is proposed, and has been proven very effective.

13. Some Questions we might ask • Q1: what Nb3Sn strands can this method be used in? • Q2: is it feasible to make practical strands? • Q3: Does the high Jc only exist in a very thin layer? • Q4: Is the refined grain size at the expense of Birr? • Q5: Can we shift the Fp-B curve peak to 0.5Birr? • Q6: Does this Hinder Margin? • Q7: Can we realize this for a multifilamentary ternary?

14. Q1: what Nb3Sn strands can use this method? Not Only Tube Conductor, but PIT, Bronze, or Internal Sn distributed barrier (e.g., RRP) – As long as a structure allows for the addition of oxide powder in a proper position, so that oxygen can be transferred, this structure is OK. PIT: Patent: Xu, Peng, Sumption, PCT/US2015/016431.

15. Q2: the feasibility of making practical strands We reported high Jc on a monofilament, but if this technique can not be used to make practical wires, then it is useless. As to the application of this method, the biggest concern lies in the drawability of Nb-Zr alloy. But this appears not be a problem for strands up to 120 stack Demonstration of making practical Tube and PIT strands: Supramagnetics’ PIT (120-sub): dsub ≈ 50 μm Hyper Tech’s PIT (61-sub): dsub ≈ 38 μm 0.4 mm 0.82 mm

16. Q3: Does the high Jc only exist in a very thin layer? 8 μm 650 C x 400h Fine grain size and associated high Jc can exist in a very thick Nb3Sn layer:

17. Q4: Is the refined grain size at the expense of Birr? Fischer, Lee, Larbalestier, PIT, 675 °C: Does refined grain size => reduced Bc2? 4.2 K Need ternary addition: Ta or Ti. An ongoing experiment: Ti doping Conclusion: introducing ZrO2 particles and refining grain size do not decrease Birr of Nb3Sn strands relative to those ordinary strands. But we do need: 1, full reaction; 2, Ta or Ti addition. With proper addition, we can push the Birr to 25-26 T.

18. Q5: Can we shift the Fp-B curve peak to 0.5Birr? 36 nm grain size => Fp-Bcurve peaks at 1/3Birr. Mechanism? Point pinning by ZrO2 particles: Fp = C∙(B/Birr)∙(1-B/Birr)2? If so, Fp-B curve peak can only shift to 1/3 of Birr. Refined grain size matching flux line spacing? If so, Fp-B curve peak can shift to 1/2 of Birr. Which is correct? To find this out, we can measure Fp-B curves of a strand with even smaller grain size. To do this, heat treated an internal oxidation strand with Nb-1Zr at 600C. (~36 nm) If ZrO2 particles 10 nm OD, they must be separated by 40 nm to get Nb-1% Zr (~45 nm) (~90 nm) An extrapolation gives: as mean grain size is reduced to ~20 nm, Bpreaches 0.5Birr. Average grain size: 30 nm

19. Q6: Is Nb3Sn T margin too small for 16 T magnets? Disturbances can cause a temperature rise, T If T > Tcs-Top, current will be shared to the matrix, and the strand will quench Since Tc(thus Tcs)  as B , any strand has less “margin” at higher B, all else being equal Question: Is our margin at 16 T too small? IWASA Q: But we know that higher Jcmakes a strand more unstable, right? A: This sort of instability is important of lower fields where J is much higher, and the energy storage (integrated heat capacity) is less than the energy due to the FJ (Jc) Of course, small filaments are needed for lower field FJ stability If at 14 T, we are at 82% Iop, then at 16 T, Tmargin is reduced by 18%. Or, to keep Tmargin, Iop/Ic reduced 4% Overall Answer: Nb3Sn high field margin is a function of B and I/Ic Higher Jc can help achieve higher margin , with some Jc going to performance, and some to lower I/Ic and thus increased margin

20. Q7: Can we realize this in a multifilamentary ternary? • We will show below our progress on multifilamentary strands • Ternaries are now being pursued, and we have several promising designs

21. Results for a Binary subelement Transport layer Jc and magnetic layer Jc of the subelements • So, • first we must translate this result into a multifilament • Then we must realize this in a ternary alloy strand

22. Factors which Influence A15 grain size in the internal oxidation method There are three key parameters which need to be optimized for a given strand (subelement) design; (1) the level of SnO2powder (2) the thickness of the Nb-Zr alloy (3) The local concentration (chemical potential) of the SnO2 (oxygen) near the Nb-Zr alloy boundary (4) the pre-HT schedule(time for oxygen injection) • The first two are important because together they set the all-important SnO2 to Nb ratio and Nb to Sn ratio. • Too little SnO2 and no oxidation, and thus no grain refinement occurs. • Too much, and the Nb as well as the Zr will be oxidized – leading to an oxygen impervious niobium-oxide barrier at the inner wall of the tube

23. Factors which Influence A15 grain size in internal oxidation method II (3) The local concentration (chemical potential) of the SnO2 (oxygen) near the Nb-Zr alloy boundary (4) the pre-HT schedule (time for oxygen injection) The third and forth are important because they set the rate, and time for, respectively, the oxygen injection into the Nb-Zr. • In fact all of the oxygen which is injected into the Nb is done before the A15 formation reaction. • To promote oxidation (and thus grain refinement) we can use; (i) more SnO2 powder, (ii) less Nb1%Zr alloy, (iii) slower ramp rates. • Because of the differences in chemical potential depending on how the SnO2 powder is deployed (a dense layer, or distributed) these ratios and optimizations are different for different strand designs

24. Simple Summary of Mono/Multi Strands

25. Thick Nb barrier 61 restack- HTR Grain size = 80 nm Jc12 T = 6800 A/mm2

26. Thin Nb barrier 61 restack- HTR Unreacted strand Grain size = 50 nm Reacted strand

27. Segregated Powders B- 61 restack- HTR Fabricated to Final size and being reacted

28. Multi-filamentary PIT strands-Supramagnetics S120B-625C/680h: The average grain size: 40 nm. 12 T Jc = 8700 A/mm2 Assuming 25% of the filaments are broken: need improvement! Transport Jc matches magnetic Jc. 220A: limit of the power supply.

29. A naive look at the limits of Jc in Nb3Sn (Rosy scenario) Engineering Jc and Icfor the five different cases: Note: Assuming all the five cases have the same Nb3Sn area fraction with the state-of-the-art RRP strands: the Nb3Sn area fraction in a subelement is 60%, the non-Cu area fraction in a strand is 0.53, the wire diameter is 0.8 mm.

30. But, how realistic? • We cannot forget to include the area needed for the powder (we will do that next slide) • Can we really push the Fpto 0.5? (It seems that we are well on our way) • Can we really make multis(it seems so, but more process development is required)

31. 4% area needed for oxide powder

32. Practical Conductor Jc and Je (12 T) • Layer Jc of present day RRP, Tube and PIT is 5000 A/mm2, with 60% A15 in subelement for RRP, and 50% for PIT and Tube • At 15 T Jc is 2700 A/mm2, Jc,nonCu (RRP) = 1600 A/mm2, and Tube/PIT 1350 A/mm2 • 4% (volume or area)is required for the SnO2 powders At 12 T • Using 1% Zr, with full oxidation, and moderate temperatures, layer Jc of 10 kA/mm2 seen at 12 T in mono • Assuming 50% fill factor, Jc,nonCu(12 T) should reach 4750 A/mm2 • If we can implement in a RRP-like design, then Jc,nonCu(12 T) should reach 5760 A/mm2in Binary

33. Practical Conductor Jc and JeII (15 T) • Layer Jcof present day RRP, Tube and PIT is 5000 A/mm2, with 60% A15 in subelement for RRP, and 50% for PIT and Tube • At 15 T Jcis 2700 A/mm2, Jc,nonCu (RRP) = 1600 A/mm2, and Tube/PIT 1350 A/mm2 • 4% (volume or area)is required for SnO2 At 15 T • Using 1% Zr, with full oxidation, and moderate temperatures, layer Jcof 3800 A/mm2 extrapolated from 14-15 T in mono • Assuming 50% fill factor and mixed powders, tube Jc,non(15 T) should reach 1805 A/mm2 (Binary) • If we used an RRP-like design, then Jc,nonCushould reach 2165 A/mm2 15 T (Binary) • With ternary push of Bc2 to 25 T, a significant further increase would be seen at 15-18 T

34. Comment and Simple Scaling Rules • While Internal Sn strands have greater A15 fractions, they may be more difficult to fabricate • Its likely that the best deff values for these new conductors will be seen with Tube and PIT • Thus, efforts in Tube and PIT are important, especially since …. • If we simply add grain refinement to a given Nb3Sn alloy (say, ternary) and do not shift the peak of Bc2, we can expect Jc to double at all fields • If we push the peak out the effects are greater still

35. So, What can we expect for a ternary Tube or PIT • If we assume pinning similar to what we have already produced, with a 1% Zr alloy, and a 35-40 nm grain size • And we use standard ternary alloying to reach Birr = 25 T • Then we expect a layer Jc = 7800 A/mm2 at 15 T • At 16 T, this should reduce by 30% to 5460 A/mm2 • With a 50% A15 fill factor, this gives 2750 • With room for the oxide, this gives 2600 A/mm2 at 16 T as a projected possible value • This does not include • (1) maximized Bc2/Birr from modified doping or optimized HTs • (2) Peak shifting from 0.35-0.5 • (3) further grain refinement (2% Zr) This gives a projected value of 2600 A/mm2, which more than meets 1500 A/mm2

36. So…. How to add the ternary? • We choose Ti, as it leads to a better strain tolerance conductor – and, it can be added to the allow after drawing • But – you can’t add it in the same area as the SnO2 – because Ti is a powerful oxygen getter SnO2 Sn+Ti Nb Pre-HT Reaction So, its possible to add it into the Nb-Zr to start – but we plan to add it afterward

37. E.Barzi--Artificial Fluxon Pinning Centers for Nb3Sn (film Test bed) • The discussion so far has been about artificial pinning of grain boundaries (or perhaps enhanced nucleation), but what about standard artificial fluxon pinning centers • This topic will also be addressed in the next talk using irradiation as a limiting case. But, in the end we want a more practical second phase pinner • Staggered Electrodeposition could be a mechanism to produce such structures (additive manufacturing of pinning centers in Nb3Sn). • Below we show an inexpensive way to produce Nb3Sn thin films on Nb, and it can be used as a test bed to try different billet materials inexpensively and with fast turnaround • Our first target: The Introduction of axial ribbons to enhance transverse component of pinning • We have to come up with ribbon materials that can be cold worked, do not dissolve at 700C and do not react with Sn. Potential candidates so far are Ta, Mo and V.

38. E.Barzi--Artificial Fluxon Pinning Centers for Nb3Sn (film Test bed) • The Nb3Sn phase is obtained by electrodeposition of Sn layers and Cu intermediate layers onto Nb substrates followed by high temperature diffusion in inert atmosphere. • Subsequent thermal treatments were realized to obtain the Nb3Sn superconductive phase.

39. E.Barzi--Artificial Fluxon Pinning Centers for Nb3Sn (film Test bed)

40. Summary and Conclusions • We have demonstrated grain refinement by a factor of 3 and a doubling of 12 T Jc in monofilaments • Internal oxidation can be used in many Nb3Sn strand types, including Tube (demonstrated) PIT (demonstrated), RRP/RIT (proposed) etc. • Sufficiently thick reaction layers can be formed • It is shown that Birris not sacrificed in present best binary strands • The Fp-B curve peak can be shifted to 0.5Birr for ultra-fine grain size • Multifilamentary strands have been demonstrated with refined grains and enhanced Jc values. These need • To be pushed to meet the even higher Jc values of their subelements • To be optimized for area fraction and Je • To be demonstrated for a ternary alloy with the ternary alloy Bc2 • This route is very promising for future Nb3Sn development

41. Xingchen Xu Recent graduate of our group PhD Thesis, OSU Materials Science Department: Prospects for Improvement in Nb3Sn conductors Refined grain and high JcNb3Sn New model for what controls stoichiometry in Nb3Sn